The present invention relates to a system for measuring the level of a fluid in an enclosed or semi-enclosed volume.
A need to continuously measure the level of a fluid in an enclosed, or semi-enclosed volume exists in numerous commercial and military applications. For example, fluid-level sensors are commonly used to monitor fluid levels in aircraft, automobiles, and trucks. Fluid-level sensors are also used to monitor fluid levels within tanks utilized for fuel dispensing, wastewater treatment, chemical storage, food processing, etc.
Electrical fluid-level sensors present safety-related issues in many applications. For example, electrical fluid-level sensors have the potential to generate sparks, and thus present an explosion hazard when used in the presence of flammable fluids. Moreover, electromagnetic interference can, in some cases, corrupt or overwhelm the signals generated by and relied upon by electrical fluid-level sensors.
Electrical fluid-level sensors commonly rely on a float mechanically or magnetically coupled to an external gauge. Alternatively, electrical fluid-level sensors can operate on the principal that the dielectric constant between a pair of wires immersed or partially immersed in a fluid changes with the fluid level. This type of sensor, however, when used to detect fuel levels, loses accuracy as the amount of contaminants, e.g., water, in the fuel increases.
The use of optical sensors to measure fluid levels in enclosed or semi-enclosed volumes has been disclosed. For example, U.S. Pat. No. 6,172,377 (Weiss), which is incorporated by reference herein in its entirety, discloses an optical fluid-level sensor comprising an optical waveguide slab. The waveguide slab includes a transparent or semi-transparent sensing rod 46 having a doping material sealed therein. The doping material, when excited by light of a first wavelength, fluoresces and emits light at a second, longer wavelength. The waveguide slab is adapted to be placed in a tank holding a volume of fluid, and is partially immersed in the fluid.
The waveguide slab is in optical communication with a light source located outside of the tank. The light source emits light at the first wavelength (hereinafter referred to as xe2x80x9cinput lightxe2x80x9d). The input light is transmitted into the waveguide slab by a bundle of optical fibers. The light is directed into the waveguide slab at an angle (hereinafter referred to as the xe2x80x9cinput anglexe2x80x9d) that causes the light to be internally reflected within the portion of the waveguide slab located above an air-fluid interface in the tank. (The desired input angle is thus a function of the index of refraction of the air surrounding the waveguide slab.) (The tern xe2x80x9cin optical communication,xe2x80x9d as used throughout the specification and claims with respect to two or more components, denotes a relationship in which optical signals can pass between the components, regardless of whether the components are physically joined or connected.)
The input light excites the doping material located above the air-fluid interface, and thereby causes the doping material to fluoresce, i.e., to emit light at the second wavelength (hereinafter referred to as the xe2x80x9coutput lightxe2x80x9d).
The waveguide slab is in optical communication with a silicon photodetector located outside of the tank. The output light is transmitted to the photodetector via a bundle of optical fibers. The photodetector generates an electrical output that is proportional to the intensity of the output light incident thereon.
The input light is travels downward through the waveguide in a zigzag pattern, and eventually reaches a location below the air-fluid interface. The input angle is chosen so that the refractive index of the fluid inhibits the internal reflection of the input light below the air-fluid interface. More particularly, the refractive index of the fluid, in conjunction with the angle at which the reflected input light strikes to interior surfaces of the waveguide, causes the input light to be transmitted out of the waveguide and into the fluid when the input light reaches the air-fluid interface. Thus, the doping material located below the air-fluid interface does not substantially fluoresce. The amount of output light reaching the silicon photodetector is therefore related to, and can be correlated with, the location of the air-fluid interface.
Optical fluid-level sensors provide substantial advantages in relation to other types of fluid-level sensors such as electrical sensors. For example, optical sensors do not require the use of electrical signals within the fluid-containing volume. Hence, optical sensors do not introduce an explosion hazard when used in conjunction with flammable fluids. Moreover, optical sensors are usually immune to the effects of electromagnetic interference, have few (if any) moving parts, and operate on relatively low amounts of power.
Fluid-level sensors are often required in space-limited applications, e.g., for fuel-level measurements in aircraft The fluid-level sensor disclosed in Weiss, while providing the substantial advantages associated with optical sensors, may not be suitable for many space-limited applications.
For example, the Weiss sensor relies on bundles of optical fibers to convey light to and from the waveguide slab. This arrangement requires that the light source and the silicon photodetector be spaced apart from the waveguide slab by a sufficient distance to avoid bending the optical fibers beyond their minimum bending radii. The need to accommodate one or more bundles of optical fibers without exceeding the minimum bending radii thereof can preclude the use of an optical fluid level sensor in certain space-limited applications. Moreover, reducing the number of optical fibers can diminish the amount of light reaching the silicon photodetector to unusable levels.
Consequently, a need exists for an optical fluid-level sensor suitable for use in space-limited applications.
A preferred embodiment of a system for measuring a level of a first fluid in a volume adapted to hold the first fluid and a second fluid located substantially above the first fluid and having a refractive index different than a refractive index of the first fluid comprises a light source adapted to produce light of a first wavelength, and an elongated optical waveguide slab in optical communication with the light source and adapted to be immersed in the first and second fluids. The optical waveguide slab comprises a material adapted to emit light of a second wavelength when exposed to the light of the first wavelength, and has a reflective surface oriented at an acute angle in relation of a longitudinal axis of the optical waveguide slab.
The preferred embodiment also comprises a waveguide housing mechanically coupled to the optical waveguide slab and being adapted to direct the light of the first wavelength into the optical waveguide slab at a selectively-variable angle so that a substantial entirety of the light of the first wavelength is internally reflected within a portion of the optical waveguide slab located above the interface of the first and second fluids, and a substantial entirety of the light of the first wavelength is transmitted into the first fluid from a portion of the optical waveguide located below the interface of the first and second fluids. The preferred embodiment further comprises a photo-multiplier tube optically communicating with the optical waveguide slab and adapted to generate an electrical output in response to the light having the second wavelength.
A preferred embodiment of a system for measuring a level of a fluid comprises a light source adapted to generate light having a predetermined wavelength, and an optical waveguide slab at least partially filled with a material adapted to fluoresce when illuminated by the light having a predetermined wavelength.
The preferred embodiment further comprises a first optical fiber in optical communication with the light source, a waveguide housing mechanically coupled to the first optical fiber, and a light pipe mounted in the waveguide housing and being in optical communication with the first optical fiber. The light pipe is adapted to transmit the light having a predetermined wavelength to the optical waveguide slab at an angle that causes the light having a predetermined wavelength to be internally reflected only within a portion of the optical waveguide slab located above the fluid thereby illuminating the fluorescent material within the portion of the optical waveguide slab located above the fluid.
The preferred embodiment also comprises a second optical fiber mechanically coupled to the photo-multiplier tube and in optical communication with the optical waveguide slab, and a photo-multiplier tube in optical communication with the optical waveguide slab and adapted to generate an electrical output in response to the fluorescence of the fluorescent material.
Another preferred embodiment of a system for measuring a level of a fluid comprises a light source adapted to generate light having a predetermined wavelength, and an optical waveguide slab at least partially filled with a material adapted to fluoresce when illuminated by the light having a predetermined wavelength.
The preferred embodiment further comprises a first optical fiber in optical communication with the light source, a waveguide housing mechanically coupled to the first optical fiber, and a light pipe mounted in the waveguide housing and being in optical communication with the first optical fiber. The light pipe is adapted to transmit the light having a predetermined wavelength to the optical waveguide slab at an angle that causes the light having a predetermined wavelength to be internally reflected only within a portion of the optical waveguide slab located above the fluid thereby illuminating the fluorescent material within the portion of the optical waveguide slab located above the fluid.
The preferred embodiment also comprises an optical detector in optical communication with the optical waveguide slab and adapted to generate an electrical output in response to the fluorescence of the fluorescent material. The waveguide housing is adapted to facilitate selective variation of a distance between ends of the first optical fiber and the optical waveguide slab.
A preferred embodiment of a system for measuring a level of a fluid in a collapsible tank having a top cover adapted to translate upwardly and downwardly in response to variations in the level of the fluid in the tank, and a base, comprises a light source adapted to generate light having a predetermined wavelength.
The preferred embodiment also comprises an optical waveguide slab at least partially filled with a material adapted to fluoresce when illuminated by the light having a predetermined wavelength. The optical waveguide is flexible and has a substantially serpentine configuration. The preferred embodiment further comprises a supporting structure for the optical waveguide slab. The supporting structure is mechanically coupled to the top cover and the base and is adapted to expand and contract in response to upward and downward movement of the top cover.
The preferred embodiment further comprises a first optical fiber in optical communication with the light source, a waveguide housing mechanically coupled to the first optical fiber, and a light pipe mounted in the waveguide housing and being in optical communication with the first optical fiber. The light pipe is adapted to transmit the light having a predetermined wavelength to the optical waveguide slab at an angle that causes the light having a predetermined wavelength to be internally reflected only within a portion of the optical waveguide slab located above the fluid thereby illuminating the fluorescent material within the portion of the optical waveguide slab located above the fluid.
The preferred embodiment also comprises a second optical fiber mechanically coupled to the photo-multiplier tube and in optical communication with the optical waveguide slab, and an optical detector in optical communication with the optical waveguide slab and adapted to generate an electrical output in response to the fluorescence of the fluorescent material.
Another preferred embodiment of a system for measuring a level of a fluid in a collapsible tank having a top cover adapted to translate upwardly and downwardly in response to variations in the level of the fluid in the tank, and a base, comprises a light source adapted to generate light having a predetermined wavelength, and a flexible optical waveguide slab at least partially filled with a material adapted to fluoresce when illuminated by the light having a predetermined wavelength.
The preferred embodiment also comprises a negator spring mechanically coupled to the optical waveguide slab and the base and biasing the optical waveguide slab so that a portion of the optical waveguide slab between the negator spring and the top cover remains in tension as the top cover translates upwardly and downwardly.
The preferred embodiment further comprises a first optical fiber in optical communication with the light source, a waveguide housing mechanically coupled to the first optical fiber, and a light pipe mounted in the waveguide housing and being in optical communication with the first optical fiber. The light pipe is adapted to transmit the light having a predetermined wavelength to the optical waveguide slab at an angle that causes the light having a predetermined wavelength to be internally reflected only within a portion of the optical waveguide slab located above the fluid thereby illuminating the fluorescent material within the portion of the optical waveguide slab located above the fluid.
The preferred embodiment further comprises a second optical fiber mechanically coupled to the photo-multiplier tube and in optical communication with the optical waveguide slab, and an optical detector in optical communication with the optical waveguide slab and adapted to generate an electrical output in response to the fluorescence of the fluorescent material.